A powertrain of a hybrid electric vehicle or a battery electric vehicle typically includes one or more high-voltage electric machines, i.e., motor generator units or electric traction motors. Each electric machine draws electrical power from a rechargeable direct current (DC) battery pack. The energized electric machines deliver motor torque, up to a total available torque as determined by a controller, to various gear sets of a transmission. A boost converter may be used in some designs to increase the battery output voltage to a level suitable for use by the electric machines.
Power inverter modules or PIMs include semiconductor switches such as IGBTs or MOSFETs that are typically controlled via pulse-width modulation (PWM) techniques. Operation of the PIM ultimately converts the boosted DC battery output voltage into an alternating current (AC) output voltage. The AC output voltage is then applied to individual phase windings of the electric machines. The energized electric machines produce the motor torque which powers the vehicle or recharges the battery pack depending on the operating mode. The semiconductor switches within the PIM can become hot during extended PWM switching operations. Therefore, a process known as derating is used to reduce power output from the PIM and thereby help protect the semiconductor switches from overheating.